Method of blowing steel melt with oxygen containing gas



rates METHOD i BLOWING STEEL MELT WITH UKYGEN CONTAINIYG GAS No Drawing.Application April 18, 1955, Serial No. 502,224. in Germany March 31,1950 Public Law 619, August 23, 1954 Patent expires March 31., 1970 5Claims. (Cl. 75-60) This invention relates to the manufacture of steelin Bessemer and Thomas converters and more particularly to a process ofblast refining which may be used to produce steel of a predetermined lownitrogen content.

This application is a continuation-in-part of prior copendingapplication of Hans (Johannes) Kosmider, Serial No. 191,729, filedOctober 23, 1950 (now abandoned), and our application Serial No.331,318, filed January 14, 1953, said application Serial No. 331,318being a continuation-in-part of our prior application Serial No.143,601, filed on February 10, 1950 (now abandoned).

It is a primary object of our invention to provide a process of blastrefining whereby steel having a substantially predetermined nitrogencontent of between 0.002% and 0.010% by weight may be produced.

The process can be applied both in the manufacture of Thomas steel andBessemer steel, and is particularly suitable for manufacture of a Thomassteel low in nitrogen content. There have already been proposeddifiierent processes for the manufacture in Thomas converters of steellow in nitrogen content. These processes consist in lowering, bysuitable measures, the solubility of nitrogen in iron. This result isobtained by proceeding to fusion at a low temperature, or by loweringthe temperature of the steel at determined instants in the course of theblowing operation. The lowering of the temperature of the molten mass,or the lowering of the temperature at which melting takes place iseffected by addition of ore, scrap iron, scale of rolling mills, orother similar fluxes.

It is also known to steel makers that towards the end of the life of aThomas converter there can be obtained, with badly worn sides andbottom, low contents of nitrogen in the course of manufacture of Thomassteel. This is due to the shorter path that the blast must traverse inpassing from the bottom of the converter through the molten bath. On thebasis that the short path taken by the current of air through the metalbath exercises a favorable effect upon the ultimate nitrogen content, ithas been proposed to introduce the refining blast of air into the metalbath at the sides. These methods of operation allow the production ofsteel low in nitrogen content, but all have the defect that theresulting nitrogen content cannot be predicted with any degree ofprecision, and in fact is subject to considerable variations. To obtaina steel having a nitrogen content of 0.010%, it is often necessary tomelt several charges by one of the above mentioned processes before thedesired nitrogen content is obtained.

With the known processes of air refining, the refining is effected bymeans of oxygen in the air blast. The oxygen of the air enters intoreaction, through the intermediary of ferrous oxide with theaccompanying elements of the pig iron, according to knownphysical-chemical laws.

In the Bessemer process, silicon, manganese, and carhas are eliminatedby the refining while in the Thomas 2,803,535 Patented Aug. 20, 1957process the combustion of phosphorous only commences after the silicon,manganese and carbon have been burned. The two processes of refining areaccomplished exothermically. When the Bessemer or Thomas process isoperated with an air blast there is blown into the iron an oxygencontaining gas consisting of approximately 79% nitrogen by volume. Thisnitrogen withdraws from the bath an appreciable quantity of heat and a.portion of the nitrogen dissolves in the liquid iron.

In the Bessemer process the nitrogen content of the iron increases to anextent dependent upon the blowing period while in the Thomas process thenitrogen content of the iron remains practically constant untl thedephosphorization stage is reached, at which time the nitrogen contentincreases sharply during the dephosphorization stage. The content ofnitrogen varies in the acid process and the basic process of blastrefining between 0.012 and 0.025%.

The harmful effect of nitrogen on the capability for deep drawing and onthe resilience of air refined steel is well known.

By practicing the present invention, there is obtained a Bessemer orThomas process for making steel in which a harmful increase of nitrogencontent is avoided and in which the converter is not exposed toexcessive thermal strain.

According to the present invention, a Bessemer or Thomas process formaking steel of a predetermined low nitrogen content from ironcontaining more than 0.2% silicon in the Bessemer process, and from ironcontaining more than 0.6% phosphorous in the Thomas process ischaracterized by the use of a blast consisting of steam and oxygencontaining gas. In the Bessemer process this blast is used during a partof or the entire blowing period while in the Thomas process the blast isemployed during at least the dephosphorizing part of the blowing period.

We have found that by using a blowing gas composed of a known proportionor mixture ratio of an oxygen containing gas and steam, that thenitrogen content of the steel is not only substantially reduced but bysuitable manipulation of the proportion of oxygen introduced into themetal melt by the blowing gas, the nitrogen content of the steel may becalculated in advance to a sufficient degree of accuracy. It will benoted that prior to our discovery, the use of steam in the blowing gaswas generally believed to be undesirable and to be avoided if possiblebecause it was thought that its presence would be deleterious to thesteel by introducing hydrogen into the steel. We have found, however,that water vapor or steam may be used without such harmful effects and,in addition, greatly reduces the cost of production by providing aneconomical substitute for previously used gases such as carbon dioxideas well as reducing the amount of commercial oxygen required. We havefound that the best results are obtained by the use of a blast gas thatcontains at least 14% by volume of steam and that a practical range ofsteam content of the blast gas is from about 14% to about 67%.

The oxygen containing gas employed with steam in our process may vary inpurity, that is the amount of pure oxygen contained in the gas, frombetween and 98% The remainder of the oxygen containing gas is substantially entirely nitrogen. In general, the mixture ratio of oxygencontaining gas to steam may vary from between one part of oxygencontaining gas to two parts of steam to six parts of oxygen containinggas to one part of steam as indicated above. The amount -of oxygen inthe mixture is obviously dependent upon the purity of the oxygencontaining gas and the proportion of steam employed.

There follow below a number of examples showing how our invention may beemployed in practice. The

term mixture ratio expresses the relation of the oxygen containing gasto steam. The term purity of the oxygen containing gas expresses thepercentage of oxygen in the oxygen containing gas. The oxygen containinggases used in the following examples are composed essentially of oxygenand nitrogen.

Example 1 Oxygen containing gas of 98% purity was used with steam in amixture ratio of 3:1 and gave a steel having a nitrogen content of0.005%.

Example 2 Oxygen containing gas of 98% purity was used with steam in amixture ratio of 1:2 and gave a steel having a nitrogen content of0.002%.

Example 3 Oxygen containing gas of 94% purity was used with steam in amixture ratio of 1:2 and gave a steel having a nitrogen content of0.003%.

The proportions of the foregoing examples can be interpolated to giveapproximate intermediate values of nitrogen in the steel or to allow forother purity values of the oxygen containing gas, the final values usedbeing readily found by trial and error. It will be clear that the degreeof purity of the oxygen also afiects the composition of the mixture usedfor blowing, since nitrogen content of the exhaust gas affects thedesired nitrogen content in the final steel. In general, it can be saidthat with reference to the indicated variables, the nitrogen content ofthe final steel in relation to the nitrogen content of waste gases canbe substantially determined. When employing the process proposed by theinvention, it is preferable to keep one of the several Variablesconstant in order to maintain the temperature variations of the moltengas within practical ranges.

As indicated from the foregoing examples, the percentage of oxygen inthe blast may vary from between about 16% to about 85%. We have found itdesirable to stay within this range since the lower percentage contentof oxygen tends to cool the metal melt while the higher concentrationsof oxygen tend to raise the temperature to impractical values. It shouldbe realized that the temperature of the metal melt may be increased byburning the waste gases such as hydrogen and carbon monoxide in asuitable manner and that the addition of scrap will cool the metalrnelt.

The conventional manner in which the blast is introduced into the metalmelt is by means of conduits which 4 connect a plurality of openings inthe converter bottom to a pressurized source of blast gas. We have foundthat a greater degree of accuracy in the final nitrogen content isobtained when the blast of gas is introduced from the sides and fromabove the metal melt.

Having described exemplary applications of our process, we claim:

1. A process for making steel having a predetermined nitrogen content ofless than 0.010 percent by blowing a metal melt in a converter with amixture of an oxygen containing gas and steam in a mixture ratio between3:1 and 1:2, the mixture containing at least oxygen and not more than 21percent nitrogen.

2. A process for making steel having a predetermined nitrogen content ofless than 0.010 percent by blowing a metal melt in a converter with ablast consisting essentially of an oxygen containing gas and steamwherein the ratio of the oxygen containing gas to steam is between 3:1and 1:2 and the blast contains at least 30% oxygen and not more than 21percent nitrogen.

3. A process for making steel having a predetermined nitrogen content ofless than 0.010 percent by blowing a metal melt in a converter for theentire conversion period with a blast consisting of an oxygen containinggas and steam wherein the ratio of the oxygen containing gas to steam isbetween 3:1 and 1:2 and the blast contains at least 30% oxygen and notmore than 21 percent nitrogen.

4. A process for making steel having a predetermined nitrogen content ofless than 0.010 percent by blowing a metal melt in a converter with ablast consisting of an oxygen containing gas and steam wherein the ratioof the oxygen containing gas to steam is between 3:1 and 1:2 and saidoxygen containing gas has an oxygen content of at least 94 percent.

5. A process for making steel having a predetermined nitrogen content ofless than 0.010 percent by blowing a metal melt in a converter for theentire conversion period with a blast consisting of an oxygen containinggas and steam wherein the ratio of the oxygen containing gas to steam isbetween 3:1 and 1:2 and the oxygen containing gas has an oxygen contentof at least 94 percent.

References Cited in the file of this patent UNiTED STATES PATENTS1,063,280 Morehead June 3, 1913 2,049,081 Salt-rick July 28, 1936FOREIGN PATENTS 583,775 Great Britain Dec. 30, 1946

1. A PROCESS FOR MAKING STEEL HAVING A PREDETERMINED NITROGEN CONTENT OFLESS THAN 0.010 PERCENT BY BLOWING METAL MELT IN A CONVERTER WITH AMIXTURE OF AN OXYGEN CONTAINING GAS AND STEAM IN A MIXTURE RATIO BETWEEN3:1 AND 1:2, THE MIXTURE CONTAINING AT LEAST 30% OXYGEN AND NOT MORETHAN 21 PERCENT NITROGEN.